Why can't I delete this face?
Convert the text to mesh using Alt+C
Switch to Face Selection as shown below





Select the face you want to remove, and choose the Knife tool to cut the part you don't want as shown below and then hit Enter:





Select the part you want to delete then hit x and choose delete Faces as shown below:





Do the same with the other faces and you will get the desired look as shown below:




Updated .blend file can be found here: